This invention relates to an improved multi-purpose zone controlled blast furnace in which the zone control concepts as originally set forth in U.S. Pat. Nos. 3,814,404, 3,928,023 are refined and extended. The improved zone controlled blast furnace of this invention can function either as a high capacity hot metal producing blast furnace (MBF) or a high capacity gas producing unit (GBF).
This invention is based on a recognition of: the blast furnace's inherent capacity to efficiently perform the multiple operations required to convert metal oxide burden materials into a quality hot metal (HM, the metallic product removed in a molten condition); its tremendous but only partially used capacity as a gas producer; the fact that expensive coke is made from rapidly diminishing raw materials in an ecologically troublesome batch process; the fact that burden permeability in the bosh where there is a counterflow of downwardly moving molten material and upwardly moving gases is a process limiting factor; the fact that the high temperature heat requirements in the bosh and a high driving rate conflict with the need for economic reduction of burden oxides in the stack; the blast furnace process is presently the least controlled of all major iron and steel making processes; the fact that the productivity of these high capital investment pieces of equipment needs to be increased; and the fact that the quality and uniformity of the product need to be improved.
The search for a better way of supplying blast furnaces with heat and reducing gas has extended over many years. Queen Elizabeth barred the use of charcoal when the forests of England started to be rapidly consumed. Coal wasn't strong enough, coke has always been too expensive, and oil and natural gas are no longer an economically attractive partial substitute. Sir Lothian Bell in the 19th century discovered the wasteful endothermic consumption of carbon (Boudouard reaction -CO.sub.2 +C=2 CO), which is called "solution loss", but neither he nor his modern counterparts have been able to economically avoid it.
To improve the process many radical design changes have been proposed e.g., U.S. Pat. Nos. 470,481, 1,704,029, 1,809,326, 1,815,899, 1,836,005, 2,795,497, 2,833,643, 2,919,185, 3,364,009, 3,594,154 and 3,778,045.
U.S. Pat. Nos. 1,289,835, 1,344,977, 1,401,222, 2,807,535, 3,189,438, 3,375,098 and 3,427,013 disclose iron ore reducing processes employing, reducing gases generated outside the shaft furnaces as a total replacement for coke, but the product produced was largely metallic iron in a non-molten form, "direct reduction iron" (DRI). Even though additional high temperature energy is required to put DRI into a molten form, it is a desired product for some uses. Some experts are now advocating its use as a blast furnace charge constituent.
Others working in several countries retained the basic blast furnace design and end product but substituted reducing gas for a portion of the coke conventionally charged. The reducing gas was formed from hydrocarbons outside the blast furnace and injected into the blast furnace stack in the bosh above where molten material is normally found. U.S. Pat. Nos. 2,727,816, 2,790,711, 2,952,533, 3,767,379, 3,811,869, 3,813,229. 3,904,398, 3,935,002 and 3,955,963 employ some combination of hydrocarbons (oil or natural gas [NG] mainly), commercial oxygen and steam to produce the reducing gas needed. U.S. Pat. Nos. 3,458,309, 3,909,446 and 3,912,502 employ combinations of natural gas, recycled top gas, and/or coke oven gas to produce the reducing gas. U.S. Pat. No. 3,767,379 describes a way of using a hydrocarbon fuel plus oxygen.
None of the methods or systems described produce a reducing gas substantially free of CO.sub.2 and H.sub.2 O, or give details concerning how the temperature, volume, and CO:H.sub.2 ratio of the reducing gas input can be regulated to economically largely eliminate "solution loss". Tests made in the U.S., Japan and Belgium demonstrated that reducing gas injected through auxiliary tuyeres (herein the term tuyeres is used broadly to include all conduits used to introduce or withdraw materials through the side of a blast furnace) located near the top of the bosh reduce "solution loss" and increase production, but the raw materials consumed in preparing the reducing gas were relatively expensive (hydrocarbons were used). Moreover, the CO.sub.2 and H.sub.2 O content of the injected reducing gas was high enough to seriously reduce its effectiveness, the CO:H.sub.2 balance was not ideal, injection temperatures were not precisely regulated near the maximum effective temperature, and the top gas produced was not converted to an efficient specification product. As a result, the savings anticipated were not realized.
U.S. Pat. No. 3,814,404 disclosed how reducing gas could be generated from low cost coals and lignite and injected in such a manner that it could create, limit and control three zones within the blast furnace, enabling them to perform the various steps of the conventional operation in an improved manner expected to decrease coke consumption "at least 20%" and provide an increase in production. Zone 1 is located in front of the primary tuyeres, zone 2 is the remainder of the bosh extending from the crucible to the mantel, and zone 3 is the portion of the furnace above the mantel. The temperature of the reducing gas at the point of injection was controlled by mixing it with cooler top gas which contained CO.sub.2 and H.sub.2 O. The patent proposed to maintain the temperature of zone 3 at or near but not over 1000.degree. C. by blending within the blast furnace, gases generated in zone 1 with lower temperature reducing gas generated externally.
This invention still further improves the operation and versatility of a zone controlled blast furnace.